A lighting device refers to a lamp that uses electricity. Depending on light emission methods, the lighting device may be divided into an incandescent electric lamp for illumination, which uses emissions of heat and light of a tungsten filament, an arc lamp for outdoor lighting, which uses incandescent light generated by arc discharge between electrodes, a fluorescent lamp, which emits light by radiating electrons to a phosphor by discharge, and the like.
Depending on purposes of use, the lighting device may be divided into a street light, a security light/guard light, a park light and the like. As for a light source for the lighting device, a sodium lamp or a mercury lamp is generally used. As the sodium lamp radiates light by using light generated by discharge of sodium vapor, its color is yellow. However, a drawback of the sodium lamp is that it takes about 20 to 30 minutes after lighting the sodium lamp for light to be sufficiently emitted, and also, its yellow color makes the sodium lamp inappropriate for use in general illumination, thereby limiting its use.
Meanwhile, the mercury lamp having high luminous efficiency normally radiates light several minutes after lighting when vapor pressure of mercury, which is initially low, is increased. However, the mercury lamp has a problem in that as temperature of the mercury lamp is decreased once light is turned off, lighting again is not possible for about 10 minutes until gas pressure is decreased.
Further, the above-mentioned lighting sources, which have fairly high power consumption, have a problem of consuming a lot of fuel in inboard power plants. For this reason, there have been efforts to apply a light emitting diode (LED) device, which has high luminous efficiency and low power consumption, to a lighting device.
However, the LED has limitations for being used for a street light, a security light, a park light and the like, due to narrowness of light radiated from the LED. In other words, a street light requires evenness to maintain uniform illumination in the street, but the narrowness of the LED provides illumination only to the areas in the vicinity thereof.
For example, the street lamp is arranged on the ground at a pre-designated interval according to its height. When a conventional sodium lamp or a mercury lamp is used as a light source, spaces between the street lamps can be sufficiently illuminated as diffusion of light is easy. However, as the LED is much narrower than the sodium lamp or the mercury lamp, spaces between the street lamps cannot be sufficiently illuminated.
In order to overcome the above problem, a conventional lighting device for a street lamp, which uses the LED as a light source, generally includes a diffusion lens, which is individually attached to a front surface of the LED, to realize targeted light distribution. However, this structure inevitably increases production resources and product costs as many lenses need to be assembled to manufacture the diffusion lens. Further, there are also problems of an increase in size and weight of products as well as optical loss of the lens itself.